Refusing to accept defeat, Vice President Biden and Mayor Bloomberg joined with three grieving Newtown families at City Hall Thursday to demand that Washington lawmakers do the right thing on gun control.

Senate leaders in Washington caved to pressure from the gun lobby this week and gave up on a proposed assault weapons ban, but there were no white flags waving at City Hall.

“Quite honestly, I’m really ashamed to see that Congress doesn’t have the guts to stand up and make a change,” said Neil Heslin, whose son Jesse Lewis was among the 20 tiny victims of the Dec. 14 Connecticut massacre.

In a poignant, emotional speech with Biden and Bloomberg by his side, Heslin, 50, recalled the morning of Dec. 14, 2012, when he saw his 6-year-old son for the last time.

He doesn’t wish that moment on any parent and challenged legislative leaders to prevent a recurrence.

“Congress, step up and make a change,” he exhorted. “There needs to be a ban on assault weapons, weapons of war, and high-capacity magazines. These weapons have no place, no use, on our streets or in our schools.”

Biden and Bloomberg started planning the Vice President’s historic visit to City Hall — the first by a Vice President in 34 years — a month ago.

The event wasn’t timed to respond to Senate Majority Leader Harry Reid’s Tuesday announcement that the main gun control bill advancing in the Senate would not include a proposed ban on assault weapons and high-capacity gun magazines.

But the setback did not seem to affect the mood — or the resolve — of the anti-gun forces in City Hall’s Blue Room.

“We really need national leadership in Washington if we’re going to stop this scourge,” Bloomberg said.

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“The only question is whether Congress will have the courage to do the right thing — or whether they will allow more innocent people, including innocent children, to be gunned down.”

Biden called on Congress to put the nation’s safety above partisan politics.

“It must be awful, being in public office and concluding that even though you might believe you should take action you can’t take action because of the political consequence you face,” Biden said.

“What a heckuva way to make a living. What a heckuva way to have to act.”

An emotional Heslin was joined by Chris and Lynn McDonnell, whose daughter Grace was killed in the Connecticut carnage, and by the family of murdered teacher Lauren Rousseau.

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“What matters is people, leaders with moral courage,” said Chris McDonnell, praising Bloomberg and Biden before his wife called on Congress to follow their example.

“We’d ask everyone who has power to influence legislation in this area, and of course those whose job it is to vote on the legislation, to ask themselves if they are doing enough,” said Lynn McDonnell.

“And if they are not, to ask themselves why.

“Why shouldn’t we be doing everything we can to bring about change?”

Reid said Thursday that the gun control bill will head to the Senate floor some time after the politicians return from a two-week recess on April 9.

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It will include measures to tighten background checks on gun buyers, to crack down on gun traffickers and to fund school safety improvements, but the assault weapons ban was stripped out because Senate leaders feared the opposition to the ban would doom the rest of the bill.

But even without the ban in the legislation, it still faces a tough climb.

Sen. Chuck Schumer (D-N.Y.) is still gathering support for the effort to require better background checks for gun buyers.

Bloomberg has used his vast wealth to press the case for new national gun restrictions, and he encouraged the nation’s voters to oust politicians who follow the lure of lobbyists over the will of the people.

“Congress has to get some courage,” he said. “It’s up to us as Americans and as fellow human beings to sort of give them that courage.”

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The mayor went to Washington last month to huddle with several top senators and with Biden, the point man on gun control for President Obama.

Bloomberg and Biden discussed holding a joint New York event during the Feb. 27 get-together. Their staffs meet weekly about gun control efforts, and City Hall keeps in touch with several Newtown families.

The three Connecticut families invited to City Hall immediately accepted the offer to appear with Bloomberg and Biden, according to an insider.

The White House launched the nation’s biggest push for new gun laws in two decades barely one month after the Newtown killings.

Biden emphasized the importance of limiting the number of bullets in an ammunition clip. The Sandy Hook shooter carried a 30-shot magazine and fired about 150 bullets in less than two minutes.

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“Think about Newtown,” Biden said. “Think of how many of these children and teachers might be alive today had (the shooter) had to reload three times as many times as he did.”

Biden argued that the proposed restrictions on weapons do not infringe on the Second Amendment right to bear arms.

“All those who say we can’t ban assault weapons, for all those who say politics is too hard, how can they say that?” asked Biden.

Bloomberg said the decision on gun control, a complicated and contentious subject, comes down to a very simple issue.

“In the end, what Congress has to decide is if it’s politically popular, or if what’s in their hearts is the right thing,” Bloomberg said.

The mayor then referenced the family members standing behind him and the losses they suffered at the hands of a madman with an assault rifle.

“What we can do as fellow human beings is try to make sure that it doesn’t happen again,” he said. “We can’t bring back their loved ones, but we can certainly do something in their memory.”